Descent Slides
Mountain Descent
Subtracting Down with Friendly Numbers
Our Mission
Today we will solve 3-digit subtraction problems by "descending" on a number line.
We will jump from the larger number down to the smaller number using Friendly Numbers.
Base Camp Warm-up
Let's practice our basic subtraction facts before we climb higher!
10 − 4
Think: 4 + ? = 10
9 − 3
Think: 3 + ? = 9
Base Camp Warm-up
8 − 5
7 − 2
10 − 7
6 − 4
9 − 5
8 − 6
Friendly Numbers
Friendly numbers are easy to work with in our heads. They usually end in 0 or 00.
Multiples of 10
50, 60, 110, 240
Multiples of 100
100, 200, 500, 900
Why use them?
It's much easier to jump from 103 to 100 than it is to jump from 103 straight to 54!
Example: 103 − 54
Starting at 103, we jump down until we reach 54.
103
54
−3
100
−40
60
−6
Total Jumps:
3 + 40 + 6 = 49
The Descent Steps
1
Put the larger number on the right side of your number line.
2
Jump back to the nearest Friendly Number (ends in 0).
3
Keep jumping by big groups until you reach the target number.
4
Add all your jump values together to find the difference.
Practice #1
152 − 88
152
88
Practice #2
214 − 135
214
135
Practice #3
305 − 247
305
247
Practice #4
440 − 165
440
165
Practice #5
512 − 394
512
394
Practice #6
625 − 489
625
489
Practice #7
731 − 558
731
558
Practice #8
803 − 676
803
676
Practice #9
918 − 745
918
745
Practice #10
999 − 856
999
856
Mission Complete
Ready to track your own trek?
"The mountain might be high, but every small jump counts!"
Mountain Trek Worksheet
Mountain Trek
Subtracting Down to the Base Camp
Name:
Date:
Trekker Instructions:
1. Start at the right side of the number line with the larger number.
2. Jump back using friendly numbers until you reach the smaller number.
3. Write each jump value above the jump.
4. Add all your jumps in the box to find the total difference.
1. 152 − 88 = _______
Total Jumps
+ + =
88
152
2. 214 − 135 = _______
Total Jumps
+ + =
135
214
3. 305 − 247 = _______
Total Jumps
+ + =
247
305
4. 440 − 165 = _______
Total Jumps
+ + =
165
440
Mountain Trek (continued)
Problems 5-10
5. 512 − 394 = _______
Total Jumps
+ + =
394
512
6. 625 − 489 = _______
Total Jumps
+ + =
489
625
7. 731 − 558 = _______
Total Jumps
+ + =
558
731
8. 803 − 676 = _______
Total Jumps
+ + =
676
803
Mountain Trek (continued)
Problems 9-10
9. 918 − 745 = _______
Total Jumps
+ + =
745
918
10. 999 − 856 = _______
Total Jumps
+ + =
856
999
Trekker Reflection
Which was your favorite "Friendly Number" to jump to today? Why?
Mountain Trek Answer Key
Mountain Trek
Teacher Answer Key
Strategy: Counting Back
Subtracting from right to left using friendly numbers.
| # | Problem | Difference | Sample Friendly Jumps |
|---|
| 1 | 152 − 88 | 64 | 2 (to 150), 60 (to 90), 2 (to 88) |
| 2 | 214 − 135 | 79 | 4 (to 210), 70 (to 140), 5 (to 135) |
| 3 | 305 − 247 | 58 | 5 (to 300), 50 (to 250), 3 (to 247) |
| 4 | 440 − 165 | 275 | 40 (to 400), 200 (to 200), 30 (to 170), 5 (to 165) |
| 5 | 512 − 394 | 118 | 12 (to 500), 100 (to 400), 6 (to 394) |
| 6 | 625 − 489 | 136 | 25 (to 600), 100 (to 500), 10 (to 490), 1 (to 489) |
| 7 | 731 − 558 | 173 | 31 (to 700), 140 (to 560), 2 (to 558) |
| 8 | 803 − 676 | 127 | 3 (to 800), 120 (to 680), 4 (to 676) |
| 9 | 918 − 745 | 173 | 18 (to 900), 150 (to 750), 5 (to 745) |
| 10 | 999 − 856 | 143 | 99 (to 900), 40 (to 860), 4 (to 856) |
Instructional Tip
Encourage students to look for the "nearest exit" to a friendly number. For example, in problem #1, jumping from 152 to 150 is a quick "-2" jump that makes the rest of the problem much more manageable. Remind them that there is more than one "right" way to choose friendly jumps, as long as they arrive at the correct target number!
